1. Field of the Invention
This invention relates generally to a device for removing nitrates from water, and this invention specifically relates to a device for removing nitrates from aquatic systems to reduce the toxicity of the water in such systems to living organisms.
2. State of the Art
Nitrate contamination is a serious problem in many aquatic systems. The primary source of dissolved nitrates is in run-off water containing nitrogenous animal waste (urine, feces, uneaten food, decayed tissues, etc.) from poultry, dairy cattle, hogs, and aquacultured species. The second largest source of nitrate contaminated water is the processing water used to clean and prepare poultry and meat food products. This water is heavily loaded with nitrates and it is typically stored for use in crop irrigation. The water is used as a crop irrigant only to the extent that the crops absorb nitrates. That it, the run-off irrigation water will still contain nitrates. Thus, the third most prominent source of nitrate contaminated water is run-off water from nitrate fertilized crop fields.
Nitrate contamination is a serious problem because nitrates are poisonous to human and animal life. Nitrogenous waste generally occurs as three highly soluble species: ammonia, nitrite, and nitrate. Ammonia is the most toxic of the three. Nitrites are known to be carcinogens and nitrates are toxic at somewhat higher levels of concentration. However, nitrates consumed by humans and animals are converted into the more toxic nitrites during digestion. Naturally occurring bacteria help to alleviate part of the problem of nitrogenous waste contamination by converting ammonia into nitrites and nitrites into nitrates. These aerobic nitrifying bacteria oxidize ammonia to nitrite, and nitrite to nitrate, respectively, as part of their metabolic and respiratory processes. Under the right conditions, i.e. anaerobic and in the presence of food molecules, nitrates become a source of oxygen to anaerobic bacteria and the nitrates are converted to Nitrogen gas.
In open aquatic systems, such as in oceans and seas, the sheer volume of water and the preponderance of nitrifying bacteria typically maintains the concentration of nitrogenous waste at safe levels. In smaller open aquatic systems which are vulnerable to land based pollution, such as lakes and aquifers which are near farms, the ecology of the aquatic system may be insufficient to maintain a low level of nitrates. In closed aquatic systems, such aquaculture ponds and aquaria, active steps must be taken to maintain safe levels of nitrogenous waste, or to avoid contamination of the water. For example, in farming areas, it is common to monitor the levels of nitrates in sources of drinking water and to avoid drinking contaminated water when the levels are too high. Pregnant women and children are often advised to drink only bottled water. The U.S. Environmental Protection Agency has set the maximum acceptable level of nitrates in drinking water at 10 ppm. Nitrates in drinking water are recognized to be the single largest causative mechanism of digestive tract cancers in humans and farm animals as well as the cause of "blue baby syndrome" in heavy agricultural regions.
Still another source of nitrate pollution exists in areas where septic tanks and water wells coexist in close proximity. This situation is quite common in New England where older septic tanks commonly leak nitrogenous waste into well aquifers rendering the water undrinkable.
The most common problem in closed aquatic systems, however, lies in aquaculture. Contaminated fishing waters pose serious economic and health problems today when demand for food fish is very high. So-called "wild" fishing waters can be contaminated by nearby or upstream sources of nitrogen, such as crop or poultry farming operations which allow nitrogenous waste to run off into a river. This has been a documented problem in the waters of the Neuse River in North Carolina, for example. There, upstream hog farming operations introduced nitrogenous waste into the river which eventually raised the level of nitrates in the downstream fishing region. The toxicity of the nitrates caused the fish to become biologically stressed and more susceptible to disease. People who consumed the diseased fish became extremely ill, and consequently, the local fisheries suffered significant economic loss. Other nitrogen polluted areas include the Everglades south of Lake Okeechobee, Fla. where the sugar industry has been a major contributor to ecological upsets. In South Florida, the most practical solution to date has been to plant nitrogen consuming crops in the affected areas, but this method is very slow acting and requires large planted tracts.
Nitrogen pollution of fishing waters has become so common, that commercial food fisheries have turned their efforts to pond aquaculture or fish farms where the conditions of the water can be more tightly controlled. However, even in these "aquarium" environments, nitrogenous waste can be a serious problem due to a high density of aquatic livestock. As used herein, the term "aquarium" refers to both relatively large fish farms containing many millions of gallons of water as well as relatively small pet fish aquaria containing thousands, hundreds, or even several gallons of water.
All multi-cellular animal life-forms in aquarium environments give off nitrogenous waste in the form of ammonia as a direct result of physiological, respiratory and metabolic activity. Most marine organisms, especially soft-bodied invertebrates such as corals that are typically found in aquarium environments, have a very low tolerance for ammonia and nitrite, and only a slightly better tolerance for nitrate. In aquaria, ammonia and nitrite reach toxic levels at approximately 0.25 parts per million (ppm), which is a level much higher than that found naturally in sea water. The level of nitrates in sea water is approximately 2-5 ppm. In the open ocean, nitrate levels are controlled by dilution and by anaerobic bacteria which consume nitrates as an oxygen source for metabolism. In closed marine systems, nitrates accumulate until their concentration is reduced by periodic water changes or by pockets of an anaerobic bacterial system. Water changes of fifteen percent every two weeks are typical. One must change the water regularly to keep the marine life alive. This necessity can be extremely expensive inland, where natural sea water is not available and purified water must be mixed with artificial sea salts. Moreover, in the case of aquaculture ponds, each pond must be completely drained at least once per year so that nitrate muck can be scraped up from the bottom of the pond. A typical commercial pond is five to ten acres, about ten feet deep, and contains approximately fifteen to thirty million gallons of water. The nitrate muck removed from these ponds is considered to be toxic waste by some regulatory agencies and disposal is a severe problem.
It is known to take advantage of bacterial anaerobic metabolic capability in order to reduce nitrate levels in aquaria. Water is pumped from the aquarium into a holding tank where a filter pad is populated with chemoautotrophic bacteria. Methanol and other nutrients are introduced into the holding tank to drive the bacteria into anaerobic metabolism and to provide a source of carbohydrates (methanol being the simplest carbohydrate). While the water is slowly circulated for several hours, the bacteria use the nitrate ions as an oxygen source to consume the carbohydrates as food. At a programmed time the water in the tank is pumped back into the aquarium and a new batch of water to be treated is obtained. Those skilled in the art will recognize that methanol is poisonous and must be used carefully in this system, lest the aquarium water be contaminated with unconsumed methanol. For this reason and other economic considerations (e.g. the cost of the methanol and the nutrients), this system is impractical for the treatment of drinking water or commercial fish pond water.
Electrolysis is presently known to be used in certain systems for the removal of certain undesirable and/or toxic substances from water. Electrolysis involves electrochemical reactions requiring at least two electrodes, usually metallic, an anode and a cathode. In general, corrosion is a very common problem when metals come in contact with water, particularly with salt water, and is particularly a problem while current flows through the electrodes. Marine life cannot tolerate abnormally high levels of metallic ions or corrosion products, particularly in a closed system. Most electrode materials currently available are not suitable for nitrate reduction in marine aquarium applications for several reasons: (1) most non-noble metals, such as copper, corrode in sea water; (2) many electrode materials, such as copper, mercury and lead, are poisonous to marine life; (3) since most noble metals are excellent catalysts for the hydrogen ion reduction reaction, the hydrogen evolution reaction will proceed at a reaction rate many times larger than the nitrate reduction reaction rate, causing the pH of the sea water to increase beyond acceptable limits for marine life survival; (4) the electrode materials result in chlorine generation which, even in extremely low concentrations, is highly toxic to marine life. The only way to prevent chlorine generation is to decrease the anodic current density by greatly increasing the anode to cathode surface area ratio, which would also result in a device of immense size. In addition, the bulk processing of nitrate in any water system requires a large electrode surface area which would result in a device of immense size.
Devices exist which use electrolysis to remove toxic substances from aqueous systems. Such devices are disclosed in U.S. Pat. No. 4,956,057 to Stucki et al., (the U.S. Pat. No. '057 patent), U.S. Pat. No. 4,212,724 to Moeglich (the U.S. Pat. No. '724 patent), U.S. Pat. No. 5,148,772 to Kirschbaum (the U.S. Pat. No. '772 patent), U.S. Pat. No. 4,257,352 to Habegger (the U.S. Pat. No. '352 patent), U.S. Pat. No. 3,891,535 to Wikey (the U.S. Pat. No. '535 patent), U.S. Pat. No. 3,542,657 to Mindler (the U.S. Pat. No. '657 patent), and in U.S. Pat. No. 4,056,482 to Schmeider (the U.S. Pat. No. '482 patent), all incorporated herein by reference. While these devices use electrolysis to a certain degree of effectiveness, some of the above-mentioned problems are inherent in them, as described below.
The U.S. Pat. No. '057 patent describes a process for removal of nitrites and nitrates from an aqueous solution by means of electrolysis. The aqueous solution is fed to a separate cathode space of the electrochemical cell. Gas formed by the electrolytic reduction, containing H.sub.2, NH.sub.3, NO and N.sub.2 O is passed through a catalyst bed producing N.sub.2 and H.sub.2 O. The formation of NH.sub.3 and H.sub.2, indicates very high current densities, which would also cause the formation of Cl.sub.2 gas from salt water, and is thus not suitable for salt water applications. As mentioned above, NH.sub.3 is poisonous to aquatic life. The formation of H.sub.2 results in a change of pH, which, if more than slight, cannot be tolerated by aquatic life.
The U.S. Pat. No. 4,212,724 patent discloses an oxidation and coagulation method and apparatus suitable for use with aquariums which includes a plurality of electrodes disposed in a chamber connected to a source of e.m.f. (A.C.) and a plurality of electrically conductive particles. The electrodes are preferably mounted by opposite side walls of the chamber so that they extend horizontally, substantially parallel to each other. Oxidation of nitrogenous waste does not remove dissolved nitrates, as nitrates can only be removed by reduction processes.
The U.S. Pat. No. 3,891,535 patent describes an aquarium water treatment apparatus utilizing two or more plates spaced apart and insulated from each other. A power source is provided for oppositely polarizing juxtaposed plates. A low voltage field is periodically reversed to prevent any buildup of impurities on the plates. Released gases, such as oxygen, may be carried to the bottom of the bodies of water to enhance the aerating effect and the sterilization of the water.
Both the U.S. Pat. No. 4,212,724 and U.S. Pat. No. 3,891,535 patents disclose the formation of large amounts of hydrogen, oxygen and chlorine, which are toxic to aquatic life and corrosive to stainless steel and other metal electrodes.
The U.S. Pat. No. '772 patent describes a method and apparatus for electrically inhibiting bacteria growth in aquariums including electrodes connected to a D.C. power source.
The U.S. Pat. No. '352 patent discloses a protozoan marine life inhibitor for use with an aquarium which includes a pair of carbon rod electrodes connected to an A.C. power source and positioned in a stream of water to terminate the protozoan life forms in the water passing between the electrodes. The U.S. Pat. No. '352 patent reveals that the exact electrical and physiological phenomena are not fully understood, but that the protozoan life forms undergo life altering experiences.
Neither the U.S. Pat. No. '772 patent nor the U.S. Pat. No. '352 patent address the issue of nitrate reduction. Further, the '772 system uses galvanized wire electrodes, which can be poisonous to marine life.
The U.S. Pat. No. '657 patent demonstrates the viability of reducing nitrates using certain metal electrodes (copper, lead, tin, iron, silver, cadmium, platinum, cobalt, nickel, and alloys thereof) which he found to be good electro-catalysts for nitrate reduction. This process has several disadvantages including heat generation, and the requirement of relatively high current densities and voltages. These disadvantages together with the unsuitability of metal electrodes make this process unsuitable for potable water treatment or aquatic life support systems. While the U.S. Pat. No. '482 patent proposes a system of nitrate reduction using graphite electrodes, the reaction requires the addition of cations such as copper, lead, or titanium in order to compensate for the poor electro-catalytic behavior of graphite. In actual practice, the graphite shown in the U.S. Pat. No. '482 patent is merely a substrate on which copper, lead, or titanium plates out and the "electrode" actually becomes copper, lead, or titanium. As mentioned above, these metals are unacceptable for use in aquatic systems.